A prepreg used in the production of a fiber-reinforced composite molded article is usually provided with its one side supported by releasable paper coated with silicone for the purpose of retaining the shape thereof. Also, the other side of the prepreg is covered with a protective film such as a polyethylene film to retain the tack of the prepreg or to prevent foreign substances such as dust from adhering thereto.
However, when one side or both sides of prepreg are covered with these sheet-like materials such as releasable paper and protective film, the resin contained in the prepreg is deviated to the sheet-like material side by the interaction such as surface tension on the sheet-like material, which makes both surfaces of the prepreg resin-rich.
The prepreg with both surfaces being resin-rich has the characteristics that even if the sheet-like material is peeled off after the prepreg is cut into a desired pattern to process the prepreg into a composite molded product to be intended, the state that the surfaces of the prepreg are resin-rich is left unchanged just after the sheet-like material is peeled off. If these prepregs with resin-rich surfaces are merely laminated in plural, air is trapped between two prepregs.
If the prepreg is cured in such a state that air is trapped between these prepregs, the obtained molded article is inferior in integration. Alternatively, the traces of the trapped air form voids, which become stress-concentrated portions under some stress and therefore the function of the prepreg is impaired: for example, the strength required for a composite molded article is obtained insufficiently.
In order to avoid such a disadvantage, it is necessary to prevent air from being trapped between the laminated prepregs. There is, for example, a method in which when laminating prepregs by using, for example, an auto lay-up machine, prepregs are pressed to each other under heating and pressure to let the air out of the space between prepregs to laminate. However, this auto lay-up machine is very expensive, which also affects the price of a resulting composite molded product. Also, this auto lay-up machine is not suitable for use in the case of laminating in a complicated form and is therefore limited in its use. For these reasons, the current operation of laminating prepregs is still actually carried out by operators manually.
Therefore, a method is proposed in which talc is stuck to the surface of a prepreg to prevent resins from being stuck plane-wise directly to each other on the surfaces of the prepregs, thereby reducing the stickiness of the resin itself to prevent air trapping. This method of sticking talc is effective to reduce tack characteristics of a prepreg. However, it merely reduces the tack characteristics, and because the talc is finally contained in a molded article, an unintended increase in weight is brought about in the case of producing a large molded product. Also, it is desirable to stick the talc evenly in consideration of the mechanical characteristics of a molded article. In this method, however, there is the case where spotting is caused to localize the talc. In this case, there is a fear that the mechanical characteristics of a molded article are deteriorated.
In another method of preventing layer voids as disclosed in theses “Production of void free composite parts without debulking” and “Degree of impregnation of prepregs—Effects on porosity” made public in the 31st (held in Apr. 7 to 10, 1986) and the 32nd (held in Apr. 6 to 9, 1987) International SAMPE Symposium respectively, a prepreg is adopted in which a reinforced-fiber sheet is not impregnated entirely with resin but one surface of the reinforced-fiber sheet is locally impregnated with the resin. It is described in these theses that this prepreg in which one surface thereof is locally impregnated with the resin can prevent the generation of voids because, when this prepreg is cured, conduits are formed which permit the escape of the air and volatile materials trapped between layers when laminating plural prepregs.
This prepreg in which one surface of a reinforced-fiber sheet is locally impregnated with the resin is produced using a method in which a reinforced-fiber sheet is interposed between releasable paper coated with the resin weighed in advance and releasable paper non-coated with the resin, followed by fastening and pressing from both sides.
Meanwhile, in a method disclosed in Japanese Patent Laid-Open Publication No. 2-227212, a number of concave grooves along the molding direction of a prepreg are formed on the surface of the prepreg impregnated sufficiently with resin by using a roller having a irregular portion in its circumferential direction. It is described in this publication that in the case of molding by vacuum bagging molding in which a plurality of the above prepregs are laminated in the condition that the direction of the concave grooves are fixed, the concave grooves on the surface of the prepreg function as conduits allowing the escape of the air and volatile materials trapped between layers and it is therefore possible to prevent the generation of layer voids.
However, in the case of the prepreg in which the resin is localized on one surface thereof and which is produced by the above double-film method, the releasable paper non-coated with the resin must be peeled off to use the prepreg in the state of a structure constituted of a prepreg and the releasable paper which is coated with resin and supports the prepreg. At this time, the releasable paper which is non-coated with the resin is in direct contact with the reinforced fibers. Therefore, when the releasable paper is peeled off, single yarns are drawn from the reinforced-fiber sheet and these drawn single yarns entangle other single yarns, resultantly bringing about the disadvantage that an opening is formed in the entangled part, which is sometimes a serious defect of a product.
Also, no resin exists on the other surface of the prepreg and therefore, when prepregs are laminated in such a manner that the sides on which no resin exists are made to face each other, they cannot be laminated in the condition that these sides are stuck to each other, leading to inferior handling characteristics of the prepreg.
When the upper side and lower side are laminated such that the upper side concave groove intersects with the lower side concave groove in the method in which the concave groove is formed on the surface of the prepreg as disclosed in Japanese Patent Laid-Open Publication No. 2-227212, the shapes of the concave grooves are kept, so that the concave grooves serve as air conduits, allowing the trapped air to flow, resulting in the production of a void-free molded article. However, when the upper and lower layers are laminated such that the concave grooves are substantially parallel to each other, the concave groove on the surface of the prepreg is easily clogged with the backside material of the prepreg laminated thereon and therefore the air is trapped on the contrary with the result that voids are easily formed.
The present invention has been made to solve the above conventional problem and it is an object of the present invention to provide a prepreg which is free from a phenomenon that air is trapped between the layers of the prepreg to generate voids when the prepregs are laminated to produce a molded product, has excellent handling characteristics and can produce a molded product having any complicated shape and also to provide a method of producing the prepreg.